Electron beam exposures have high precision but have a disadvantage of low throughput. Various technologies have been developed in order to eliminate this disadvantage. At the present, a technology referred to as "character projection" exposure has come into practical use. In a character projection exposure, a semiconductor circuit pattern is transferred using a mask on which is formed a plurality of various repeating small pattern elements ("features") (on the order of 10 .mu.m square). However, in order to apply this character projection to an exposure of a real semiconductor integrated circuit (such as a DRAM) onto a wafer, the throughput is low (on the order of one digit).
In contrast, as a type of photolithography apparatus that imprints an integrated circuit pattern onto a semiconductor wafer, an electron beam "reduction-transfer" apparatus is known that irradiates an electron beam onto a mask defining a fixed pattern, and then reduces and transfers the image of the pattern of that irradiated region to a wafer utilizing a two-stage projection lens (for example refer to Kokai Hei 5-160012). Because the electron beam cannot collectively irradiate the entire mask region in this type of apparatus, the field of the optical system is divided into many small regions (termed "main fields" and "subfields"), and an image of each small region is transferred while the state of the electron optical system changes for each small region (for example refer to U.S. Pat. No. 5,260,151).
Further, other methods involving use of a MOL (Moving Objective Lens) or a VAL (Variable Axis Lens) are known. These methods shift the axis of the lens by means of applying a magnetic field (generated by an axis-shifting deflector) to the lens magnetic field. However, these types of full-scale reduction-transfer apparatus are still under development.
In a conventional electron-beam transfer-exposure method, the state of each pattern portion and of each layer (regarded as the same category) which will undergo pattern formation was ignored, and the fundamental conditions of the electron optical system were considered to be always the same. For example, in an electron optical system that has VAL or MOL lenses, it was assumed that the VAL or MOL would work in the same way for a contact hole layer or a layer of a pattern that has a small filling factor as for a pattern wiring layer or a layer of a pattern having a large filling factor. Furthermore, the dimensions of the main field and of the subfields were fixed without deviations for the contact hole layer or a layer of a pattern having a small filling factor as well as for the pattern wiring layer or a layer of a pattern that has a large filling factor. Even further, the dimensions of the main field and of the subfields were fixed without any deviations for layers which required high precision or for layers whose comparative precision was not as severe.
As a result, under conditions that could achieve sufficient precision for layers which required high precision (critical layers), pattern formation would occur with excessive precision (too precise) for layers whose comparative precision is not as severe, resulting in the throughput of the apparatus being reduced on the whole.
Moreover, a beam current of the electron beam sufficient for forming a contact hole layer or a layer of a pattern having a small filling factor causes a problem when used to form a pattern wiring layer or a layer of a pattern having a large filling factor. However, an exposure method suitable for both layers was not thought out (for example, distinguishing between SMD (symmetric magnetic doublet) and MOL, VAL). This also caused mismatch between the precision in each pattern portion and the throughput.